Alternative pre-mRNA splicing is a major mechanism for regulation of mammalian gene expression, and has a demonstrated role in the regulation of cancer cell proliferation, apoptosis, and invasiveness. Thus, it is likely that tumor progression correlates with and may be partially caused by changes in alternative splicing regulation. Preliminary research suggests that the regulation of FGFR2 and Enah exon skipping events is linked to mammary tumor progression. I therefore propose to characterize the regulation of FGFR2 exon lllb and Enah exon 11a splicing in mouse mammary cell lines and WAP-Myc mice using fluorescent alternative splicing reporters. Fluorescent alternative splicing reporters allow for in situ visualization of the splicing decisions made by individual cells in developmental and pathological microenvironments. The proposed experiments use evolutionary conservation of cis pre-mRNA elements and a high-throughput cell-based genetic screen to identify and characterize regulatory factors for FGFR2 and Enah in a rapid, unbiased manner, but return to a whole animal model of tumor progression to validate biological significance. PUBLIC HEALTH RELEVANCE: Careful study of the expression of genes that regulate cell shape and responses to growth factors in normal and cancerous mouse mammary cells may advance our understanding of gene expression during tumorigenesis and metastasis. This investigation may identify novel biomarkers and therapeutic targets for breast cancer.